Integrated solar combined cycle systems (ISCCS)

The integration of a solar field with a combined cycle (CC) power plant seems to offer several advantages to solar power and also to other renewable energy sources. However, careful thought should be given to particular design features in order to circumvent sizeable, inherent green energy losses. The CC is an optimised system and upon integration with solar it is rendered out of optimum and may lose some percentage of its inherent high efficiency. For instance, it occurs upon combining two or more power-block subsystems, which operate with different capacities and varying fuel injection modes. Every percent of CC efficiency-loss due to the integration induces a considerably magnified effect on the green energy output of the system. For example, consider a 100 MW CC operating 8000 hours a year, to be integrated with a solar trough power plant capable of operating 2000 hours a year at 10 MW output (when operating alone with its own 10 MW engine). When the latter is integrated with the CC, a single percent loss of the CC annual output amounts to 8 GWh. This loss may be said to "eat up” a substantial portion of the 20 GWh solar plant green capability output. Any efficiency loss of a fuel fired engine is a green energy loss, because now more fuel has to be burnt to supply the same amount of the electricity prior to the integration.

In addition, another loss takes place during the solar hours. As the temperature of the working fluid produced by the solar field is often too low for accommodating the design conditions of the steam-cycle part of the CC, "back-up” fuel firing is generally resorted to, which is a severe, inefficient use of fuel [4]. It means a definite green energy loss. Every single percent of CC efficiency loss will be magnified as mentioned above, and is to be charged to the solar energy account.

Also, because site requirements for the solar field and the CC are in conflict, this brings about another source of annual CC output loss. The CC conversion efficiency is degraded due to the usual higher ambient temperature (and lower air density) at high solar radiation sites, which are beneficial to solar systems [4]. This might translate into additional percentage of solar green output loss.

There may be ISCCS or other solar-fuel hybrids configurations that make sense, but significant care and study must be taken in the integration design, otherwise it is possible to end up with a plant that uses more fuel per kWhe output than the fossil-only plant. Green energy considerations and analysis should help.

The role of the standard reference for defining avoided fuel (emissions reduction) or green energy is essential for enabling fruitful analysis of systems and for directing research options towards improved systems. It should be emphasized that in most cases the standards (references) are independent of the equipment characteristics of the system under examination.